1. Field of the Invention
This invention relates to a method and an apparatus for welding glass sheets together to make a multiple glazed unit.
2. Discussion of the Prior Art
In the manufacture of double glazed window units, it is well known that such units can be made by uniting the margins of an assembly of glass sheets with a continuous peripheral weld, the central regions of the sheets which are bound by the continuous weld being pulled apart while the welding periphery is soft to establish a desired spacing between the sheets. In the process of manufacturing such all-glass multiple glazed units, two glass sheets are first carefully washed, dried, preheated and assembled at a welding station one above the other. An electrically conductive stripe is deposited on one of the sheets, generally on the upper surface of the upper glass sheet, to form a continuous peripheral electrical path. A plurality of electrodes, preferably placed at the corners of a rectangular window unit, are provided to direct the flow of electrical heating currents through selected portions of the stripe causing it to be heated. The portions of the upper sheet underlying the stripe are heated by the current flowing through the stripe until the glass obtains a temperature at which the stripe burns off. At this stage, the heated margins will have attained a temperature at which the glass is electrically conductive so that heating current from the electrodes now flows through the heated margins.
The corresponding margins of the lower sheet are heated by their close association with the upper sheet and as the heating continues the margins of the upper sheet soften and sag into contacting relationship with the margins of the lower sheet. Heating is continued until the margins of both sheets are softened and run together to form a continuous peripheral weld uniting the assembled sheets. Thereafter the sheets are pulled while the air is moved between the sheets in a known manner to bloom the edges of the unit.
Various modes of controlling the heating current supplied to the edges of the glass sheet have been utilized in the prior art. For example, in U.S. Pat. No. 2,398,360 the applied current is either supplied to all four edges simultaneously or in a step-by-step fashion to one side after another by switching the current from one pair of electrodes to another pair. In U.S. Pat. Nos. 3,510,285 and 3,796,556 a switching current is permitted to flow alternately first through one pair of opposed edges of the glass sheets and then through the other pair of opposed edges.
In U.S. Pat. No. 3,628,935 the relay operation of U.S. Pat. No. 3,510,285 is eliminated and a saturable reactor circuit applies a polyphase voltage first to one pair of opposite edges and then the remaining pair of opposite edges. U.S. Pat. No. 3,726,658 provides an electrode arrangement whereby the current flows between diagonally opposed pairs of electrodes, the diagonals being shifted periodically to change the flow pattern, while heating all four edges of the glass sheet simultaneously.
U.S. Pat. No. 3,847,584 teaches that welding current is applied to each of the side edges in turn of a one of the glass sheets in a first heating cycle. The current applied to each edge is automatically controlled through a series of four or more potentiometers which are selected in a timed sequence to produce a desired pattern of current flow and thus a predetermined heating pattern in the selected edge whereby the edges are gradually and uniformly brought up to a desired temperature.
The prior art teachings in general can be grouped into two categories, namely (1) those which teach the sequential heating of the margins or marginal edge portions of a glass sheet and (2) those which teach applying a voltage to opposed corners of a glass sheet. Although each of these techniques are satisfactory for their intended purposes, there are limitations. For example, in the practice of sequentially heating the marginal edges, the procedure is slow because current is individually applied to each margin edge until the glass attains the fusing temperature. Further, the last heated edge is hotter than the first heated edge. The temperature difference between the first heated edge and the last heated edge depends on welding time and/or glass size. As the welding time for each edge increases and/or the length of the edge increases, the temperature difference increases. Further, as the temperature difference increases, the thickness of the weld varies. This is because the hotter glass edge is more viscous and easily shaped whereas the cooler glass edge is less viscous and more difficult to shape. Nonuniformity of edge thickness decreases edge quality.
The limitation in practicing the diagonal welding technique is that the conductive stripe does not conduct current uniformly because the stripe is not applied uniformly. More particularly, the thickness and/or width of the conductive stripe sometimes varies, and the electrical conductivity of the stripe varies. The variation of electrical conductivity results in higher heating currents passing through one pair of adjacent edges than through the other pair of adjacent edges. After the stripe is burned off, the pair of adjacent glass edges are at different temperatures and have different electrical resistivity. The resultant weld is nonuniform for similar reasons discussed above.
As can be appreciated, it would be advantageous, therefore, to provide a process of and apparatus for welding edges of glass sheet that does not have the drawbacks of the prior art.